Optical branching device

Abstract: An optical branching device comprising: a substrate (201); a first core member formed on the substrate, having a first edge face (211C); a second core member (220) formed on the substrate, tapering toward the first edge face of the first core member, having a second edge face (220C) facing the first edge face at a given space (B); and a third core member (221) formed on the substrate, tapering toward the first edge face of the first core member, having a third edge face (221C) facing the first edge face at a given space (B).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical component available for use in the field of optical communication, optical signal processing and the like, and particularly to an optical branching device or branch waveguide. Related Background Art In the fields of optical communication and,optical signal processing, waveguide type optical branching devices are often utilized responding to demands for miniaturization of optical components. A representative optical branching device is described in Japanese Patent Application Laid-open No. HEI 5-11130 (11130/1993). Also, directional coupler is described in Journal of Lightwave Technology 10(2): 1843-1849, December 1992. SUMMARY OF THE INVENTION Conventional optical branching devices have no satisfactory small insertion loss. The optical branching device of the present invention has adequate processing stability, and further offers a lowered loss at the branching section. The optical branching device according to the present invention comprises a substrate; a cladding member disposed on said substrate; a first waveguide embedded in said cladding member and having an optical axis and an end face, said end face being simultaneously substantially perpendicular to said optical axis and to an interface between said substrate and said cladding member; and second and third waveguides embedded in said cladding member; said second and third waveguides being separated from one another, and each of said second and third waveguides tapering toward and contacting directly said end face of the first waveguide. In the above defined optical branching device said first waveguide lowers the energy density of light to be input thereinto and makes the wave front of said light parallel to said end face of said first waveguide. The present invention can also be on optical component comprising a plurality of the optical branching device. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of an optical branching device according to an embodiment of the present invention; Fig. 2A is a section view of the device shown in Fig. 1, taken along the plane indicated by the arrows H-H in the drawing; Fig. 2B is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows B-B in the drawing; Fig. 2C is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows C-C in the drawing; Fig. 2D is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows D-D in the drawing; Fig. 2E is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows E-E in the drawing; Fig. 2F is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows F-F in the drawing; Fig. 2G is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows G-G in the drawing; Fig. 3 is an illustration of the optical branching device shown in Fig. 2A; Fig. 4 is a section view of a modification of the optical branching device shown in Fig. 2A with improvements made on the edges 220d, 221d of the second core member 220 and third core member 221; Fig. 5 is a section view of an optical branching device assembly with a 1 x 4 structure which consists of 3 connected optical branching devices BR201, BR202 and BR203 of the type shown in Fig. 4; Fig. 6 is a section view of an optical branching assembly with a 1 x 8 structure which consists of 7 connected optical branching devices BR201, BR202, BR203, BR204, BR205, BR206 and BR207 of the type shown in Fig. 4; Fig. 7 is an illustration of another optical branching device of the invention wherein two branch waveguides are contiguous with a non-branch waveguide; Fig. 8 is a perspective view of an directional coupler; Fig. 9 is a section view of the device shown in Fig. 8, taken along the plane indicated by the arrows H-H in Fig. 8; Fig. 9A is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows A-A in the drawing; Fig. 9B is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows B-B in the drawing; Fig. 9C is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows C-C in the drawing; Fig. 9D is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows D-D in the drawing; Fig. 9E is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows E-E in the drawing; Fig. 10 is a horizontal section view of a Y-shaped waveguide type optical branching device (Y-shaped device); Fig. 11 is a section view of another waveguide type optical branching device; Fig. 12 is a section view of an optical branching device which is a modification of the optical branching device shown in Fig. 11, wherein the joints 1070 are replaced by a linear multimode waveguide 1060; DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention and related optical branching devices will now be explained in detail with reference to the accompanying drawings. Incidentally, like elements in the drawings will be indicated by like identical reference characters, avoiding repeated explanation. The present inventors have made a study of light waveguides for a long time. The following is an explanation regarding several types of optical branching devices well known in their laboratory. Fig. 10 is a horizontal section view of a Y-shaped waveguide type optical branching device (Y-shaped device). The optical branching device shown in Fig. 10 will be referred to as type A waveguide. Type A waveguide comprises a substrate 1001, straight single mode waveguide 1011 formed on the substrate 1001, a multimode tapered waveguide 1012 connected to the waveguide 1011, and single mode waveguides 1020, 1021 which are branch waveguides connected to the waveguide 1012. That is, the Y-shaped device shown in Fig. 10 (type A) is a device for branching and coupling guided light transmitted via the single mode waveguide 1011 or the single mode waveguides 1020, 102, by passing through the multimode waveguide 1012. Fig. 11 is a section view of another type of waveguide type optical branching device. The optical branching device shown in Fig. 11 will be referred to as type B waveguide. Type B waveguide comprises a substrate 1001, and single mode waveguides 1071, 1072 formed on the substrate 1001. The single mode waveguides 1071, 1072 are located close and in parallel at the joints. The present inventors have found that these type A, type B waveguides should be improved from the following points of view. To fabricate type B device with a desired branching ratio, the process step must be controlled so that the space very precisely matching the designed size is provided between the joints 1070 of the waveguides 1071, 1072. The present inventors prepared some samples of type B waveguide. As a result, some defectives were prepared. For this reason, the inventors diagnosed the defectives. Microscopic observation of the samples revealed that the space between the joints 1070 of the waveguides 1071, 1072 did not have the designed size. The present inventors believe that one reason for this accident is incomplete etching during the preparation of the joints 1070. Yields of the device of Fig. 11 when produced on a large scale should be increased. The waveguide of Fig. 12 is an optical branching device fabricated by replacing the joints 1070 of the directional coupler of Fig. 11 by a straight multimode waveguide 1060. Connected to the respective terminals are bent single mode waveguides 1040, 1041 or 1042, 1043. The waveguide shown in Fig. 12 will be referred to as type C waveguide. Type C device exhibits excellent processing stability because it has no joining structure of type B device wherein two single mode waveguides are located close in parallel. Accordingly, type C waveguide is produced in higher yields than type B waveguide. The present inventors have further noted that in the Y-shaped device of Fig. 10, the tip of the sharp gap section 80 at the branching section tends to become round, not sharp. This change causes an increase in the loss of light at the branching section. This loss in light lowers the yields of conforming products. In an attempt to increase the coupling efficiency of the electric field distribution at the side of multimode waveguide 1060, and the same distribution at the side of the single mode waveguides 1040-1043 at the branching section, the present inventors reduced the size of the distance of the single mode waveguides on a trial basis. The results were, however, that the waveguide constituents (core materials) invaded the gap section 1080 near the branching section, causing rounding (deformation). Rounding of the gap section 1080 brings about discontinuity of the mode and increases the coupling loss. Excessively reduced gap between the single mode waveguides contributes to an increased coupling loss. With conventional preparation technology, it is difficult to have the gap reduced to one micrometer or less. Therefore, excess reduction in the gap is not preferable from the point of view of improving the processing stability and yields. As noted above, for types A and D waveguides, the lowering of the loss and the improvement in processing stability are essential. The waveguide type optical branching device of the present invention is provided as a waveguide type optical branching device with satisfactory processing stability, and further reduced loss at the branching section. An explanation will now be made of an optical branching device according to an embodiment of the present invention. Fig. 1 is a perspective view of an optical branching device according to an embodiment of the present invention. Fig. 2A is a section view of the device shown in Fig. 1, taken along the plane indicated by the arrows H-H in the drawing. Fig. 2B is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows B-B in the drawing. Fig. 2C is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows C-C in the drawing. Fig. 2D is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows D-D in the drawing. Fig. 2E is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows E-E in the drawing. Fig. 2F is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows F-F in the drawing. Fig. 2G is a section view of the device shown in Fig. 2A, taken along the plane indicated by the arrows G-G in the drawing. Fig. 3 is a section view of the device shown in Fig. 2A. The waveguide type optical branching device according to the present embodiment comprises a silicon substrate 201, a cladding member 202 formed on the supporting substrate 201, a first core member (first tapered waveguide) 210, a second core member (second waveguide) 220, and a third core member (third waveguide) 221. The first core member 210 is embedded in the cladding member 202. The second core member 220 is embedded in the cladding member 202 as well. Also, the second core member 220 in contiguous with a first end face 211c of the first core member 210. The interface (second end face) between the first end face 211c and the second core member 220 is indicated with 220c. The third core member 221 is also embedded in the cladding member 202. The third core member 221 is contiguous with a first end face 211c of the first core member 210. The interface (third end face) between the first end face 211c and the third core member 221 is indicated with 220c. The cladding member 202 (cladding 2, cladding layer 202) has a first surface 202a. The first surface 202a is parallel to the main surface 201a of the substrate 201. The main surface 201a is defined by an interface between the substrate 201 and cladding member 202. "Direction of the width" is defined to be the direction perpendicular to both the direction perpendicular to the first surface 202a, and to the direction of light propagation. "Direction of the thickness" is defined to be the direction perpendicular to the first surface 202a. The first core member 210 has a light transmission path 212 of given width and thickness, and a core section 211, which is contiguous with the light transmission path 212, of a tapered type whose width increases as the distance from the light transmission path 212 does. The width of the light transmission path 212 is defined by the distance between the side 212a and side 212b in Fig. 2A. The first core member 210 has a first perpendicular section 212d which is defined by intersection with a first plane (plane indicated by the arrows B-B in Fig. 2A) which is perpendicular to the first surface 202a. The first core member 210 has a second perpendicular section 212e which is defined by intersection with a second plane (plane indicated by the arrows C-C in Fig. 2A) perpendicular to the first surface 202a and located between the fist end face 211c and first plane (B-B plane). The first core member 210 has a first horizontal section (210a in Fig. 2A) which is defined by intersection with a third plane (plane indicated the arrows H-H in Fig. 1) parallel to the first surface 202a. The second core member 220 has a third perpendicular section 220e which is defined by intersection with a fourth plane (plane indicated by the arrows E-E in Fig. 2A) which is perpendidular to the first surface 202a. The second core member 220 has a second horizontal section (220f in Fig. 2A) which is defined by intersection with the third plane (H-H plane). The third core member 221 has a fourth perpendicular section 221e which is defined by intersection with the fourth plane (E-E plane). The third core member 221 has a third horizontal section 221f which is defined by intersection with the third plane (H-H plane). In other words, the branch waveguide assembly shown in Figs. 1-3 comprises a substrate 1, a first tapered waveguide 210, a second waveguide 220 and a third waveguide 221. The second core member 220 tapers toward the first core member 210. The third core member 221 tapers toward the first core member 210 as well. In addition, the distance between the second core member 220 and third core member 221 lowers toward the first core member 210. In this optical branching device, the first core member 210 includes a first multimode waveguide 210, while the second core member 220 includes a first single mode waveguide 220, and the third core member 221 includes a second single mode waveguide 221. The first core member 210 comprises a tapered waveguide 211. Since both the second core member 220 and third core member 221 are tapered, if the space between the second core member 220 and third core member 221 is constant, the optical axis OP220 of the second core member 220 at the second end face 220c, and the optical axis of OP221 the third core member 221 at the third end face 221c may be situated closely. Accordingly, at the second end face 220c of the second core member 220, the light transmitted in this second core member 220 has an electric field distribution in the radial direction of the core 220. The peak position of the electric field distribution (the first peak position) corresponds to the position of the axis OP220 of the second core member 220. At the third end face 221c of the third core member 221, the light transmitted in this third core member 221c has an electric field distribution in the radial direction of the core 221. The peak position of the electric field distribution (the second peak position) corresponds to the position of the axis of the third core member 221. With the first peak and the second peak situated closely, improvements are obtained in the coupling efficiency of the light transmitted between the first core member 210 and second core member 220 as well as in the coupling efficiency of the light transmitted between the first core member 210 and third core member 221. As a result, according to the present invention, it is possible to input guided light which has been branched or coupled, with a minimum loss at the branching section. The second core member 220 comprises a second end face (interface) 220c, third perpendicular section (220e) and second horizontal section 220f. The second end face 220c opposes the first end face 211c of the first core member (first tapered waveguide) 210. The third perpendicular section 220e is defined by intersection with a fourth plane (E-E plane) perpendicular to the first surface 202a. The second horizontal section 220f is defined by intersection with the third plane (H-H plane). Here, since the second core member 220 tapers, the area of the second end face 220c is smaller than the area of the third perpendicular section 220e. The second core member 220 comprises a section 220h which is defined by intersection with the plane (G-G plane) parallel to the fourth plane. The third perpendicular section 220e is situated between the interface 220c and the section 220h. The area of the section 220h is larger than the area of the third perpendicular section 220e. The third core member 221 is embedded in the cladding member 202. The third core member 221 comprises a third end face 221c, fourth perpendicular section 221e and third horizontal section 221f. The third end face (interface) 221c opposes the first end face 211c of the first core member 210. The fourth perpendicular section 221e is defined by intersection of the third core member 221 with a fourth plane (E-E plane). The third horizontal section 221f is defined by intersection of the third core member 221 with the third plate (H-H). As the third core member 221 tapers, the area of the third edge face 221c is smaller than the area of the fourth perpendicular section 221e. The third core member 221 comprises a section 221h which is defined by intersection with the plane (G-G plane) parallel to the fourth plane (E-E). The fourth perpendicular section 221e is situated between the interface 221c and the section 221h. The area of the section 221h is larger than the area of the fourth perpendicular section 221e. The substrate 201 has a substrate surface 201a. The first tapered waveguide 210 is formed on the substrate surface 201a, and has a first end face 211c. The second waveguide 220 is formed on the substrate surface 201a. The second waveguide 220 has an interface 220c with the first end face 211c, a first side 220a including a plane crossing the substrate surface 201a, and a second side 220b facing the first side 220a. The third waveguide 221 is formed on the substrate surface 201a. The third waveguide 221 has an interface 221c with the first end face 211c, a third side 221a including the place crossing the substrate surface 201a, and a fourth side 221b facing the third side 221a. The fourth side 221b is located between the first side 220a and the third side 221a. The first side 220a is located between the second side 220b and the fourth side 221b. The width of the first end face 211c is identical to the length of the first end face 211c along the direction (direction of the width) perpendicular to both the direction of the normal to the substrate surface 201a (direction of the thickness), and the direction of the normal to the first end face 211c (direction of the optical axis). The width (Wt) of the first end face 211c is greater than the distance between the second side 220c and the third side 221c. The first core member 210 is located so that the light input into the first core member 210 through the interface 212c is propagated in the direction along the first horizontal section 210a across the first perpendicular section 212d and second perpendicular section 212e, and output from the first end face 211c. The second core member is located so that the light output from the first end face 211c is input into the second core member 220 through the second interface 220c, and is propagated in the direction along the second horizontal section 220f across the third perpendicular section 220e, and output at the second end face 220d of the second core member 220. The third core member 221 is located so that the light output from the first end face 211c is input into the third core member 221 through the third end face 221c, and is propagated in the direction along the third horizontal section 221f across the fourth perpendicular section 221e, and output from the third core member 221. In this connection, the area of the first end face 211c is larger than the area of the first perpendicular section 212d. This is because the first core member 210 has a tapered section 211 which diverges toward the direction of the second and third core members 220, 221. The tapered section 211 has sides 211a, 211b which forms given angles with the optical axis OP (center line) of the first core member 210. In other words, the intersections 211a, 211b between the two sides perpendicular to the first surface 202a of the tapered section (tapered waveguide) 211, and the third plane (H-H plane) form given angles with the optical axis OP (center line ) of the first core member 210 in the third plane. As the first core member 210 comprises the tapered section 211, the density of the energy (power) of the optical signal input into the first core member 210 at the end face (fourth end face) 212c decreases gradually as the light travels passing through the first perpendicular section 212d toward the first end face 211c. The light output from the end face 211c of the first core member 210 is input into the second core member 220 at the second end face 220c, and into the third core member 221 at the third end face 221c. The present inventors have found, based on their previous research, that for the improvement in coupling efficiency of light propagated between two optical components it is effective to make the opposing end faces of the respective optical components perpendicular to the direction of light propagation. The first end face 211c according to the present embodiment is perpendicular to the direction of movement of the light passing through the first end face 211c. Therefore, the interface 220c is perpendicular to the direction of movement of the incident light to the interface 220c. The interface 221c is perpendicular to the direction of movement of the incident light to the interface 221c. As a result, the first end face 211c is opposite to the interface 220c, whereas the first end face 211c is parallel to the interface 220c. The first end face 211c is opposite to the interface 221c, whereas the first end face 211c is parallel to the third end face 221c. The second core member 220 has sides 220a, 220b perpendicular to the first surface 202a. The side 220a and side 220b are parallel to the direction of movement of the light propagated in the second core member 220. In this connection, the side 220a and side 220b face each other, while the side 220a and side 220b runs in parallel with each other. Accordingly, the sides 220a and side 220b are perpendicular to both the first surface 202a and the second edge face 220b. Similarly, the third core member 221 has sides 221a, 221b which are perpendicular to the first surface 202a. The side 221a and side 221b are parallel to the direction of movement of the light propagated in the third core member 221. In turn, the side 221a and side 221b are opposite to each other, whereas the side 221a and side 221b are parallel to each other. Thus, the side 221a and side 221b are perpendicular to both the first surface 202a and third end face 221c. The second waveguide 220 comprises a waveguide of constant width 220x which has a given width, and a waveguide of variable width 220y which is contiguous with the waveguide of constant width 220x. The waveguide of variable width 220y tapers toward the first end face 211c, and the width of the waveguide of variable width 220y ranges from 1/2 to 4/5 the width of the waveguide of constant width 220x. The third waveguide 221 comprises a waveguide of constant width 221x which has a given width, and a waveguide of variable width 221y which is contiguous with the waveguide of constant width 221x. The waveguide of variable width 221 tapers toward the first edge face 211c, and the width of the waveguide of variable width 221y ranges from 1/2 to 4/5 the width of the waveguide of constant width 221x. An intersection (cross line) 220i is an intersection of the second interface 220c and second side face 220b. An intersection (cross line) 221i is an intersection of the third interface 221c and third side face 221a. An intersection (cross line) 220j is an intersection of the second interface 220c and first side face 220a. An intersection (cross line) 221j is an intersection of the third interface 221c and fourth side face 221b. An intersection (cross line) 211i is an intersection of the first edge face 211c and the side face 211b. An intersection (cross line) 211j is an intersection of the first edge face 211c and the side face 211a. The distance between the line 211i and line 211j is Wt (µm), the distance between the line 220i and line 221j is (2W + A) (µm), distance Wt is greater than (2W + A). The distance (A) is defined by the space between the second core member 220 and third core member 221. More precisely, the distance (A) is defined by the distance between the line 220j and line 221j. The distance between the second core member 220 and the third core member 221 increases with distance from the first core member 210. This is because the second core member 220 comprises bent sides 230a, 230b bending apart from the third core member 221. The bent side 230a is adjacent to the plane 220a, whereas the bent side 230b to the plane 220b. The curvature radius of the bent side 230a is R, and also the curvature radius of the bent side 230b approximates R. The third core member 221 has bent sides 231a, 231b bending apart from the second core member 220. The bent side 231a is contiguous with the plane 221a, whereas the bent side 231b to the plane 221b. The curvature radius of the bent side 231a is R, and also the curvature radius of the bent side 231b approximates R. The second core member 220 and the third core member 221 are S-shaped waveguides. The light output from the second edge face 220c and input into the second core member 220 is then output through the end face 220d of the second core member 220. The light output from the third end face 221c and input into the third core member 221 is then output through the end face 221d of the third core member 221. In this connection, the optical distance from the input end face 220c to the output end face 220d of the second core member 220 is equal to the optical distance between the input end face 221c and the output end face 221d of the third core member 221. As a result, in cases where beams of the light input from the first core member 210 into the core members 220, 221 are output from the core members 220, 221, the beams of the light branched by the core members 220, 221 are in the same phase upon output from the end faces 220d, 221d. Since the beams of the output light from the end faces 220d, 221d are in one phase, the phase matching of the respective beams are easy to perform for coupling of the output light, etc. Fig. 4 shows a modification of the optical branching device shown in Fig. 2A with improvements made on the end faces 220d, 221d of the second core member 220 and third core member 221. The end faces 220d, 221d of the optical branching device shown in Fig. 2A is exposed. The direction of the normal to the exposed end face 220d forms a given angle with the direction of movement of the light (direction of the optical axis) propagated in the third core member 221. Also, the direction of the normal to the exposed end face 221d forms a given angle with the direction of movement of the light propagated in the third core member 221. As mentioned above, the present inventors have noted, based on their previous research, that for the improvement in coupling efficiency of light propagated between two optical components it is effective to make the opposing end faces of the respective optical components perpendicular to the direction of light propagation (direction of the optical axis). For this finding, as shown in Fig. 4, the optical branching device according to the present embodiment is positioned so that the output end face 220d of the second core member 220 is perpendicular to the optical axis OP202 (indicated by an alternating long and short dash line) of the second core member 220. Optical fiber F202 is located opposite to the end face 220d of the second core member 220 via a lens L202. Additionally, the third core member 221 is located so that the output end face 221d of the third core member 221 is perpendicular to the optical axis OP203 (indicated by an alternating long and short dash line) of the third core member 221. Optical fiber F203 is located opposite to the end face 221d of the third core member 221 via a lens L203. Additionally, the first core member 210 is located so that the input end face 212c of the first core member 210 is perpendicular to the optical axis OP201 (indicated by an alternating long and short dash line) of the first core member 221. Optical fiber F201 is located opposite to the end face 212c of the first core member 210 via a lens L201. Fig. 5 is an optical branching device assembly with a 1 x 4 structure which consists of 3 connected optical branching devices BR201, BR202 and BR203 of the type shown in Fig. 4. This optical branching device assembly comprises the first optical branching device BR201, the second optical branching device BR202 whose input end face 212c is connected to the output end face 220d of the first optical branching device BR201, and the third optical branching device BR203 whose input end face 212c is connected to the output end face 221d of the first optical branching device BR201. The optical signal (indicated by a solid arrow in the drawing) input into the first optical branching device BR201 through the end face P201 is divided with this optical branching device, and output through the end faces P202, P203 of the second optical branching device BR202, and through the end faces P204, P205 of the third optical branching device BR203. Conversely, the respective optical signals (indicated by alternating long and short dash arrows) input at the edge faces P202-P205 are coupled with the optical branching device, and output from the end face P201. Fig. 6 is an optical branching device assembly with a 1 x 8 structure which consists of 7 connected optical branching devices BR201, BR202, BR203, BR204, BR205, BR206 and BR207 of the type shown in Fig. 4. This optical branching device assembly comprises the first optical branching device BR201 with an input end face (input port), the second optical branching device BR202 whose input end face 12c is connected to the output end face 220d of the first optical branching device BR201, and the third optical branching device BR203 whose input end face 212c is connected to the output end face 221d of the first optical branching device BR201. In addition, this optical branching device assembly comprises the fourth optical branching device BR204 whose input end face 212c is connected to the output end face 220d of the second optical branching device BR202, the fifth optical branching device BR205 whose input end face 212c is connected to the output end face 221d of the second optical branching device BR201, the sixth optical branching device BR206 whose input end face 212c is connected to the output end face 220d of the third optical branching device BR203, and the seventh optical branching device BR207 whose input end face 212c is connected to the output end face 221d of the third optical branching device BR203. Accordingly, this optical branching device assembly may branch single light beam input into the assembly, into eight beams, and reversely may couple eight beams input into this assembly into single beam. In this connection, all the optical branching devices BR201, BR202, BR203, BR204, BR205, BR206, and BR207 are formed on the same substrate 101. Fig. 7 illustrates another optical branching device wherein a second core member 420 and third core member 421 are contiguous with the first core member 410. Hereunder, the directional coupler according to the present invention will be explained. Fig. 8 is a perspective view of a directional coupler according to the present invention. Fig. 9 is a section view of the device shown in Fig. 8, taken along the plane indicated by the arrows H-H in Fig 8. Fig. 9A is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows A-A in the drawing. Fig. 9B is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows B-B in the drawing. Fig. 9C is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows C-C in the drawing. Fig. 9D is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows D-D in the drawing. Fig. 9E is a section view of the device shown in Fig. 9, taken along the plane indicated by the arrows E-E in the drawing. The directional coupler according to the present embodiment comprises a silicon substrate 501, a cladding member 502 formed on the supporting substrate 501, a first core member 560, a second core member (second waveguide) 520, and a third core member (third waveguide) 521. The first core member 560 is embedded in the cladding member 502. The second core member 520 is embedded in the cladding member 502. The second core member 520 comprises an interface (second edge face) 520c which is contiguous with the first end face 511c of the first core member 560. The third core member 521 is also embedded in the cladding member 502. The third core member 521 comprises an interface (third end face) 521c which is contiguous with the first end face 511c of the first core member 560. The first core member 510 comprises a fourth end face 511d facing the first end face 511c. The fourth core member 540 is embedded in the cladding member 502. The fourth core member 540 comprises an interface (fifth end face) which is contiguous with the fourth end face 511c of the first core member 560. Also, the fifth core member 502 is embedded in the cladding member 502. The fifth core member 541 comprises an interface 521c (sixth end face) which is contiguous with the fourth end face 511d of the first core member 560. The cladding member 502 (cladding 502, cladding layer 502) has a first surface 502a. The first surface 502a is parallel to the main surface 501a of the substrate 501. "Direction of the width" is defined to be the direction perpendicular to both the direction perpendicular to the first surface 502a, and to the direction of light propagation. "Direction of the thickness" is defined to be the direction perpendicular to the first surface 502a. The first core member 560 has given width and thickness. The first core member 560 has a first perpendicular section 512d which is defined by intersection with a first plane (plane indicated by the arrows B-B in Fig. 9) which is perpendicular to the first surface 502a. The width of the first perpendicular section 512d is constant. The first core member 560 has a first horizontal section (560a in Fig. 9) which is defined by intersection with a third plane (plane indicated the arrows H-H in Fig. 8) parallel to the first surface 502a. The second core member 520 has a third perpendicular section 520e which is defined by intersection with a fourth plane (plane indicated by the arrows E-E in Fig. 9) which is perpendicular to the first surface 102a. The second core member 520 has a second horizontal section (520f in Fig. 9) which is defined by intersection with the third plane (H-H plane). The third core member 521 has a fourth perpendicular section 521e which is defined by intersection with the fourth plane (E-E plane). The third core member 521 has a third horizontal section 521f which is defined by intersection with the third plane (H-H plane). The fourth core member 540 has a fifth perpendicular section 540e which is defined by intersection with a fifth plane (plane indicated by the arrows A-A in Fig. 9) which is perpendicular to the first surface 102a. The fourth core member 540 has a fourth horizontal section (540f in Fig. 9) which is defined by intersection with the third plane (H-H plane). The fifth core member 541 has a sixth perpendicular section 541e which is defined by intersection with the fifth plane (A-A plane). The fifth core member 541 has a fifth horizontal section 541f which is defined by intersection with the third plane (H-H plane). In other words, the branch waveguide shown in Figs. 8 - 9E comprises a substrate 501, straight waveguide (mixer) 560, second waveguide 520, third waveguide 521, fourth waveguide 540, and fifth waveguide 541. The width of the first end face 511c is the length of the first end face 511c along the direction (direction of the width) perpendicular to both the direction of the normal to the substrate surface 501a (direction of the thickness), and the direction of the normal to the first end face 511c (direction of the optical axis). The width of the first end face 511c is greater than the distance between the second side 520c and the third side 521c. The width of the first end face 511c is greater than the distance between the second side 520b and the third side 521a. The width of the second waveguide 520 lowers toward the mixer 560. Accordingly, the area of the section 520e is greater than the area of the second end face 520c. The width of the third waveguide 521 lowers toward the mixer 560. Accordingly, the area of the section 521e is greater than the area of the third end face 521c. The width of the fourth waveguide 540 lowers toward the mixer 560. Accordingly, the area of the section 540e is greater than the area of the fifth end face 540c. The width of the fifth waveguide 541 lowers toward the mixer 560. Accordingly, the area of the section 521e is greater than the area of the sixth end face 521c. The first core member 560 is positioned so that the light input into the first core member 560 via the fourth waveguide 540 and fifth waveguide 541, is output from the optical branching device through the second waveguide 520 and third waveguide 521. The directional coupler shown in Fig. 9 comprises, in the same manner as the Y-shaped device of the present embodiment, a core formed on a silicon substrate 501-that is, single mode waveguides 520, 521, 540, 541, and a multimode waveguide 560, and a cladding 502 with the core embedded therein. To either terminal 511c of the multimode waveguide 560 there are connected the single mode waveguides 520, 521, while the single mode waveguides 540, 541 are connected to the other terminal 511d. The single mode waveguides 520, 521, 540, and 541 are located close at a space A, and are positioned symmetrically with respect to the center line (optical axis) of the multimode waveguide 560. Above described A is minimum distance between the second core member 520 and 521, or minimum distance between the second interface 520c and third interface 521c. The space between the single mode waveguides 520, 521 increases with distance from the first core member 560. The space between the single mode waveguides 540, 541 increases with distance from the first core member 560. The single mode waveguides 520, 521 are identical to the branch waveguides 220, 221 of the Y-shaped device shown in Fig. 1. These waveguides comprise proximal ends of constant width from which tapering sections extend smoothly toward the multimode waveguide, and connect to single mode waveguides 520y, 521y with peripheral curvature radius R. The proximal ends 520x, 521x are bent waveguides with width W1, and curvature radius R. The widths of the terminals 520c, 521c of the single mode waveguides 520y, 521y at the side of the multimode waveguide 560 are W2. Proximal ends 540x, 541y of constant width taper gently toward the multimode waveguide 569, and are connected to single mode waveguides 540y, 541y with peripheral curvature radius R. The proximal ends 540x, 541x are bent waveguides with width W1, and curvature radius R. The widths of the terminals 540c, 541c of the single mode waveguides 540y, 541y at the side of the multimode waveguide 560 are W2. The waveguide widths of the proximal ends 520x, 521x, 540x and 541x (= W1) are set to meet the following relation: (3/8) π ≤ V ≤ (1/2) π In this type directional coupler, the widths of the terminals 520c, 521c, 540c, 541c of the single mode waveguides 520, 521, 540, 541, which are branch waveguides, at the side of the multimode waveguide 560 are smaller than the widths (= W1) of the proximal ends 520x, 521x, 540x, 541x. That is, a characteristic aspect resides in the fact that the widths W2 of the terminals 520c, 521c, 540c, 541c of the single mode waveguides 520, 521, 540, 541 is smaller than at the side of the multimode waveguide 560 are smaller than the widths W1 of the proximal ends 520x, 521x, 540x, 541x. With this reduced widths, the directional coupler shown in Fig. 9 have shortened spaces between the centers of the single mode waveguides 520, 521, 540, 541 which contribute to establishment of favorable processing stability. Of course, the coupling efficiency of guided light at the branching section of this optical branching device is improved as compared with conventional ones. As a result, the loss of guided light resulting from branching or coupling with the directional coupler of the present embodiment is smaller than that experienced with the directional coupler shown in Fig. 12. Additionally, in order to both ensure the significant effects of the reduced terminal widths which far overweigh the increased radiation loss, and maintain enough processing stability, the widths (W2) of the terminals 520c, 521c, 540c, 541c of the single mode waveguides 520, 521, 540, 541 are desired to be 1/2 - 4/5 the widths (= W1) of the proximal ends 520x, 521x, 540x, 541x. In short, (W1) x (1/2) ≤ W2 ≤ (W1) x (4/5) In conclusion, the directional coupler according to the present invention may be used suitably for the construction of an optical measurement system which sweeps light over a rather wide wavelength range. The present invention is not limited to the embodiments described above, and may be modified in various ways. For example, so long as the structure where the terminal width of the single mode branch waveguide is smaller than the proximal width, is maintained, the composition of the waveguide type optical branching device, forms and dimensions of the respective parts thereof, etc. are not limited to those of the above-noted embodiments. In addition, the configuration of the waveguide type optical branching device is not limited to the embedded configuration of the present embodiments, and any other configuration including ridge type, loading type, etc. may be used. Furthermore, the single mode branch waveguides need not be placed symmetrically with respect to the center line of the multimode waveguide. The present inventors believe that the same excellent processing stability and loss reduction as noted above may be accomplished even with asymmetrical optical branching devices. Y-shaped devices and directional couplers were referred to as examples of optical coupling devices which further include star couplers, and all these devices are within the scope of the present invention so long as they have the branching structure characteristic of the present invention. Furthermore, a plurality of the optical branching devices of the present invention may be combined to form a waveguide type device assembly by situating the output terminal of each waveguide so as to connect to the input terminal of the waveguide positioned directly downstream therefrom. With the resulting waveguide type device assembly, the respective constituent devices produce satisfactory effects in a cumulative manner, thereby allowing drastic suppression of the loss while keeping excellent processing stability, for the output of guided light which has been subjected to branching or coupling. Another characteristic aspect of the present invention is in the reduced space between the centers of two single mode waveguides which is a fruit of provision of single mode waveguides with terminals less wide than the proximal ends, at the side of the multimode waveguide, and waveguide type devices with this characteristic structure fall within the scope of the present invention.

Claims: An optical branching device comprising a substrate (201, 501); a cladding member (202, 402, 502) disposed on said substrate (201, 501); a first waveguide (210, 410, 560) embedded in said cladding member (202, 402, 502) and having an optical axis (OP) and an end face (211c, 511c), said end face being simultaneously substantially perpendicular to said optical axis (OP) and to an interface (201a, 501a) between said substrate (201, 501) and said cladding member (202, 402, 502); and second and third waveguides (220, 221; 420, 421; 520, 521) embedded in said cladding member (202, 402, 502); characterized in that said second and third waveguides (220, 221; 420, 421; 520, 521) are separated from one another, and each of said second and third waveguides tapers toward and contacts directly said end face (211c, 511c) of the first waveguide (210, 410, 560). An optical branching device according to Claim 1 wherein said first waveguide (210, 410, 560) is arranged so as to lower the energy density of input light and to make the wave front of said light parallel to said end face (211c, 511c) of the first waveguide. An optical component comprising a plurality of the optical branching device of Claim 1.

Inventors: MATSUURA YUJI, KANAMORI, HIROO, MATSUURA, YUJI, SASAOKA, EISUKE, MATSUURA, YUJI, C/O YOKOHAMA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1994
Main classification: G02B6
IPC codes: G02B6